Brassica species are increasingly important oilseed crops in many parts of the world. As a source of vegetable oil, Brassica presently ranks behind only soybeans and palm and is virtually tied with sunflowers for the number three position of commercial importance. The oil is used both as a salad oil and as a cooking oil throughout much of the world.
In its original form Brassica oil, often known as rapeseed oil, was found to have deleterious effects on human health due to its relatively high level of erucic acid which commonly is present in native cultivars in concentrations of 30 to 50 percent by weight based upon the total fatty acid content. Plant scientists identified a source of low erucic acid germplasm and used this germplasm to breed low erucic acid cultivars (Chapter 6 entitled “The Development of Improved Rapeseed Cultivars” by B. R. Stefansson from “High and Low Erucic Acid Rapeseed Oils” edited by John K. G. Kramer, Frank D. Sauer, and Wallace J. Pigden, Academic Press Canada (1983)).
In Canada, plant scientists focused their efforts on creating “double-low” varieties which were low in erucic acid in the oil and low in glucosinolates in the solid meal remaining after oil extraction (i.e., an erucic acid content of less than 2 percent by weight based upon the total fatty acid content, and a glucosinolate content of less than 30 micromoles per gram of the oil-free meal). These higher quality forms of rape developed in Canada are known as canola. In contrast, European scientists worked to achieve only “single-low” types which were low in erucic acid, but did not attempt to improve the quality of the solid meal which retained a glucosinolate content of about 100 micromoles per gram of oil-free meal.
The result of this major change in the fatty acid composition of rapeseed oil was the creation of a new oil profile which often contained approximately 8 to 15 percent by weight of alpha-linolenic acid, approximately 62 percent by weight of oleic acid based upon the total fatty acid content, and approximately 7 percent or more by weight of saturated fatty acids in the form of lauric acid (C12:0), myristic acid (C14:0), palmitic acid (C16:0), stearic acid (C18:0), arachidic acid (C20:0), behenic acid (C22:0) and lignoceric acid (C24:0) based upon the total fatty acid content. Since the overall percentage of oil in the seed did not change appreciably when the new low erucic cultivars were developed, it appeared that the erucic acid oil component had been redirected into other fatty acids within the oil (Chapter 7 entitled “The Introduction of Low Erucic Acid Rapeseed Varieties Into Canadian Production” by J. K. Daun from the previously identified Academic Press Canada (1983) publication; “Prospects for the Development of Rapeseed (B. napus L.) With Improved Linoleic and Linolenic Acid Content” by N. N. Roy and A. W. Tarr, (1987) Plant Breeding 98:89-96; and “Genetic Control of Fatty Acid Composition in Oilseed Crops” by R. K. Downey and D. G. Dorrell, Proc. Flax Inst. U.S.A. 47(3)1-3).
Canola oil presently consists of approximately 7 percent saturated fatty acids primarily in the form of stearic acid (C18:0) and palmitic acid (C16:0), approximately 62 percent by weight oleic acid (C18:1) which contains a single double bond per molecule, approximately 21 percent by weight linoleic acid (C18:2) which contains two double bonds per molecule, approximately 10 percent by weight linolenic acid (C18:3) which contains three double bonds per molecule, and less than one percent by weight erucic acid (C22:1) which contains a single double bond per molecule.
Over the years scientists have attempted to improve the fatty acids profile for canola oil (for example, Chapter 10 by Gerhard Röbbelen entitled “Changes and Limitations of Breeding for Improved Polyenic Fatty Acids Content in Rapeseed” from “Biotechnology for the Oils and Fats Industry” edited by Colin Ratledge, Peter Dawson, and James Rattray, American Oil Chemists' Society (1984)). Further, scientists have been attempting to increase the overall oil content of the seed. FIG. 1 shows the steady rise in oil content of Canadian canola cultivars over the past 20 years.
In addition to high oil, the plant must also exhibit optimum agronomic performance. Such agronomic performance includes excellent vigor, flowering propensity, number of pods per plant, number of seeds per pod, plant yield, disease resistance and herbicide resistance. In order to produce high yielding lines that can compete with current commercial lines, hybrid performance is required. High oil content in the seed combined with a high yield per hectare, makes possible a very high oil yield per hectare.
The production of Brassica hybrids is challenging because Brassica plants, and in particular Brassica napus plants, are generally able to self pollinate, as both male and female sexual organs are present in each flower. Accordingly, a hybrid system is required. There are several hybrid systems available in Brassica, each with advantages and disadvantages. These include: (i) self incompatibility (SI), (ii) genetic male sterility (GMS), and (iii) cytoplasmic male sterility (CMS). In addition, there are several CMS systems available in Brassica, the most common being Ogura CMS.
In CMS systems, including Ogura CMS, the female line (A line) is male sterile by virtue of a mutation in the DNA of the mitochondria. The male line (also called the restorer line or R line) contains a restorer gene in the nuclear genome that restores male fertility in Ogura CMS plants. Restorer lines for Ogura CMS lines were originally available from Institut National de Recherche Agrocole (INRA) of Rennes, France (WO92/05251 and WO98/027806, which are herein incorporated by reference). A third line, the maintainer line or B line, is required to propagate the male sterile female line. This line is generally isogenic to the male sterile female line and differs only in the cytoplasm. A hybrid plant is produced when the female CMS line is pollinated by the male restorer line and seed is harvested from the female line. Accordingly, the genotype and the phenotype of the resulting hybrid seed and plant are determined by the genetics of the female and male parents. If the male restorer plant comprises a homozygous restorer gene, then every F1 hybrid seed will be fully restored and fertile. If the male restorer plant comprises a heterozygous restorer gene, then 50% of the F1 hybrid seed will be fully restored and fertile and 50% will be male sterile.
In Canada, Brassica grain is used primarily for oil production, as approximately 40-45% of the crushed seed is oil. There have been many attempts to alter the fatty acid profile of the oil as well as to increase the overall oil content. A higher oil yielding line could exact a premium for growers. Enhancing the oil content while simultaneously improving grain yield and agronomic traits is a major challenge for breeders (D. Hauska, C. Oertel, L. Alpmann, D. Stelling, H. Busch (2007) In Proceeding of 12th International Rapeseed Congress. Science Press USA Inc. NJ 08852, USA. pp 159-162). However, the combination of very high oil, very high grain yield, excellent agronomic performance and disease resistance in a cultivar has not been found in Brassica napus in nature, despite many years of evolution. In addition, the combination of high oil and excellent agronomic performance has not been developed in Brassica napus by man, despite over 35 years of active canola breeding in Western Canada, Europe, and Australia.